Micro-projection devices include pico-projectors and pocket projectors. The smaller, lighter pico-projectors are primarily integrated with mobile devices, e.g. cell phones, ultra-mobile PCs, and digital still cameras, and powered by the batteries of the mobile devices, but there are also independent pico-projectors. Pocket projectors are similar to traditional ones, only smaller and lighter.
Pico-projectors measuring only a few centimeters wide and a few micrometers thick have been developed to be embedded on cell phones. A pico-projector needs one dual-axial scan component or two single-axial scan components to project to a two-dimensional screen by laser scan. There are roughly two types of dual-axial scan projection: raster scan and Lissajous scan. The two axes are generally a fast axis and a slow axis. The slow axis of a raster scan must be kept at 60 Hz and the fast axis is usually above 18 kHz to attain high-quality, high-resolution, and non-flickering projected screens. It is impossible for the resonant frequency of a scan component to be as low as 60 Hz; hence the component is forced-actuated quasi-statically to vibrate at that frequency.
      θ    =          T      K        ,
The quasi-static force is related to the scanning angle by where T is the torque caused by the driving force, K is the stiffness of the twisting axis, and θ is the scanning angle. The greater the force, the larger the angle. The scanning angle can also be increased by lowering K and sacrificing the strength of the component, where the twisting axis is easily broken. A large K, however, implies increased driving force and power consumption, despite strengthening the component.
The slow axis of a Lissajous scan is not limited to 60 Hz, but configurable according to the scan structure. When the actuator drives at the resonant frequency, the scanning angle is expressed as
      θ    =          Q      ⁢              T        K              ,where Q is the quality factor of the component. Q is greater than 1500 for a common scan component made of silicon. Resonance, therefore, significantly magnifies vibrational displacement, achieving large-angle scanning even with a smaller force and lower power consumption (50 mW for instance). Consequently a Lissajous scan component can be configured to different resonant frequencies according to need and is thus more flexible in structural design.
In short, a raster scan component consumes more power but generates projections with parallel lines, less flicker, and higher scan line coverage. A Lissajous scan component, driven vertically and horizontally at resonant frequencies, consumes less power, but is less predictable with its complex scan trace, which calls for additional analysis and research. Moreover, whether a Lissajous scan screen flickers is a question of the scan line density and the frequency ratio.